Diagnosis
In Customizing the assignment of an asset is set as obligatory.
An asset assignment has not been defined.
An asset can be assigned to a building in two ways:
directly, by specifying the asset to the building.
indirectly, through reference to a higher-level building whose assetassignment is also valid for its lower-level buildings.

System Response
You cannot save the building if the asset assignment is missing.

Procedure
Define a direct or indirect asset assignment.